Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have already been put into practical use as a battery for small electronic devices such as notebook computers and cellular phones because of their advantages such as high energy density, small self-discharge, and excellent long-term reliability. In recent years, nonaqueous electrolyte secondary batteries have been more and more utilized for a battery for electrical vehicles, a battery for household use, and a battery for power storage.
A lithium ion secondary battery includes a positive electrode primarily comprising a positive electrode active material and a negative electrode containing a material capable of intercalating and deintercalating a lithium ion as a main component, and a nonaqueous electrolytic solution. Examples of positive electrode active materials used for a positive electrode include lithium metal oxides such as LiCoO2, LiMnO2, LiNiO2, LiFePO4, and LiMn2O4.
Examples of negative electrode active materials used for a negative electrode include: metal lithium; and silicon, oxides such as silicon oxides, and carbonaceous materials each of which is capable of intercalating and deintercalating a lithium ion. In particular, lithium ion secondary batteries with a carbonaceous material capable of intercalating and deintercalating a lithium ion such as graphite (artificial graphite, natural graphite) and coke have already been put in practical use.
Examples of nonaqueous electrolytic solutions used include a solution obtained by adding a lithium salt such as LiPF6, LiBF4, LiN(SO2CF3)2, LiN(SO2C2F5)2, and lithium bis(oxalate)borate(LiB(C2O4)2) to a mixed solvent of a cyclic carbonate solvent such as ethylene carbonate and propylene carbonate, and a linear carbonate solvent such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.
In a secondary battery using such a nonaqueous electrolytic solution, for example, a solvent in the electrolytic solution is reduced and decomposed on the surface of a negative electrode, especially under a high temperature environment, and the decomposition product deposits on the surface of the negative electrode to increase the resistance, or a gas generated through the decomposition of the solvent causes the battery to swell. On the surface of a positive electrode, the solvent is oxidized and decomposed, and the decomposition product deposits on the surface of the positive electrode to increase the resistance, or a gas generated through the decomposition of the solvent causes the battery to swell. As a result, the storage characteristics of a battery under a high temperature environment and the cycle characteristics of a secondary battery are lowered, which disadvantageously causes degradation of battery characteristics.
To prevent the occurrence of such problems, a compound having a function to form a protective film is added into a nonaqueous electrolytic solution. Specifically, it is known that the compound added into an electrolytic solution is intentionally allowed to decompose on the surface of an electrode active material in initial charging so that the decomposition product forms a protective film having a protective function to prevent further decomposition of a solvent, or an SEI (Solid Electrolyte Interface). It has been reported that the protective film formed on the surface of an electrode suitably suppresses the chemical reaction or decomposition of a solvent on the surface of an electrode, and as a result exerts an effect of maintaining the battery characteristics of a secondary battery (Non Patent Literature 1). Addition of, for example, vinylene carbonate, fluoroethylene carbonate, or maleic anhydride as an additive for formation of such a protective film to an electrolytic solution has been attempted to improve battery characteristics (Non Patent Literature 1).
On the other hand, addition of a boron compound to an electrolytic solution to reduce capacity degradation due to repeated charging/discharging is known.
For example, Patent Literature 1 describes use of a fluorinated boron compound selected from the group consisting of BF3, a BF3 complex, HBF4, and an HBF4 complex as an additive for an electrolyte in a nonaqueous lithium battery. In addition, the patent literature describes BF3-diethyl carbonate complex, BF3-ethyl methyl carbonate complex, and the like as the BF3 complex, and describes BF4-diethyl carbonate complex as the HBF4 complex.
Patent Literature 2 describes blending a boron compound in an electrolytic solution with a first lithium salt dissolved in a nonaqueous solvent, together with a lithium salt of an organic acid as a second lithium salt. The patent literature describes boron trifluoride and halogenated boron complexes as the boron compound, and describes boron trifluoride-diethyl ether complex, boron trifluoride-di-n-butyl ether complex, and boron trifluoride-tetrahydrofuran complex as the halogenated boron complex.